It is known to determine various characteristics of a fluid from the measurement of electromagnetic or acoustic properties of the fluid. For example, WO2009/118569 discloses a Robinson oscillator sensor apparatus comprising a cavity member which contains a fluid and defines a resonant cavity for an electromagnetic field that extends into the fluid. The sensor apparatus may be used for identifying the fluid and/or determining a composition of the fluid according to the Robinson principle in which knowledge of both a resonant frequency and an electromagnetic loss in the cavity may provide an indication of whether a property of a fluid present in the cavity is within a prescribed parameter range regardless of the quantity of fluid present in the cavity. Knowledge of both the resonant frequency and loss may provide an indication of the quantity of fluid present in the cavity regardless of whether a property of a fluid present in the cavity is known to be within a prescribed range of parameters.
In such known methods for determining the characteristics of the fluid, the cavity member and/or coupling elements for coupling signals between RF electronic components of the sensor and the electromagnetic field may not operate reliably or may lack robustness in a demanding environment such as a downhole environment. For example, entrained solids or particulates in the fluid may clog or damage the cavity member and/or the coupling elements thereby impairing measurement sensitivity. For hydrocarbon fluids, deposition of contaminants or substances such as hydrates on the cavity member and/or the coupling elements may also impair measurement sensitivity. Such problems may be exacerbated by high fluid pressures and/or temperatures. Cavity members and/or coupling elements that extend into the cavity may also present an obstruction to fluid flow through the sensor. This may block or at least partially restrict the movement in or through the cavity of particulates, solids and/or the like entrained within the fluid. Additionally or alternatively, this may lead to an undesirable drop in fluid pressure.